The present invention relates to telephone circuits for feeding a subscriber's telephone line. More particularly, the present invention relates to monolithically integratable circuits designed to be inserted in a telephone interface circuit between a subscriber's telephone line and exchange control components.
In telephone exchanges of the electromechanical type which are still in operation, in which the control components drive the line devices by means of inductive couplings devices, a subscriber's telephone line is generally supplied via a circuit comprising two identical resistances R.sub.DC which couples the wires of the line, via a resistance in each case, to two terminals of a constant supply voltage generator V.sub.BAT. In most cases, this generator takes the form of a 48V battery.
The voltage V.sub.L supplied to the subscriber's telephone line, expressed as a function of the line current I.sub.L, which is determined by the line load, is therefore: EQU V.sub.L =V.sub.BAT -2R.sub.DC I.sub.L.
This relationship defines the external current-voltage characteristic of the supply circuit at its line connection terminals.
The characteristic of the circuit, shown in Cartesian form, taking the values of the line current I.sub.L as the abscissa and the values of the voltage V.sub.L supplied to the line as the ordinate, for positive abscissa and ordinate values, is therefore a section, of a straight line having the relationship mentioned above as its equation and is shown in FIG. 1 of the drawings by a dashed line segment.
If the line current is zero, then the voltage V.sub.L is equal to the battery voltage V.sub.BAT.
The slope of the characteristic of line 1 with respect to the abscissa axis is determined by the overall value 2R.sub.DC of the resistances via which the line is coupled to the terminals of the supply voltage generator V.sub.BAT. By selecting, or devising, a specific current-voltage characteristic for a telephone circuit designed to supply a subscriber's telephone line, whose load R.sub.L increases with increases in its length, it is possible to determine the impedance of the circuit seen from the line and ensure that the line current I.sub.L has, as a function of the various line lengths contained in the entire telephone network, a value lying between predetermined values, and it is also possible to accurately predetermined the values of the power dissipated at the exchange as a function of the line length. The characteristic of line 1 is not only completely adequate for normal telephone network requirements, even though it is provided using an extremely simple circuit, but is also the most advantageous external characteristic for a telephone supply circuit since it enables full use of the voltage V.sub.BAT supplied by the supply voltage generator for the maximum line lengths.
The introduction of partially or fully electronic telephone exchanges, in which functions previously carried out in subscriber's sets were carried out by means or electronic circuits inserted in interface circuits in series with the line itself when the subscriber's line was connected, led to the problem of supplying these electronic circuits in addition to the subscriber's line. In effect, an electronic circuit requires a minimum operating voltage, whose value increases, inter alia, with an increase in the intensity of the current flowing in the circuit itself, since, if the exchange comprises electronic circuits inserted in series with a subscriber's telephone line, it must be borne in mind that a portion V.sub.DROP of the supply voltage V.sub.BAT may not be supplied to the line as it is necessary for the operation of these circuits.
Persons skilled in the art currently design both the electronic circuits to be inserted in series with a subscriber's telephone line, and the supply circuits for exchanges of an electronic type, in such a way that it is possible to maintain a characteristic which is still rectilinear for these supply circuits with an impedance which is equal to the impedance provided on the line by a non-electronic circuit for exchanges of the electromechanical type, while attempting to reduce the inevitable V.sub.DROP as much as possible.
A characteristic of the type shown in FIG. 1 by the line segment 2 which parallel to the characteristic of line 1 and located as close as possible thereto is consequently used.
Although the characteristic of line 2 entails higher overall design costs, it enables the achievement of electronic supply circuits which are compatible, from the point of view of the line, with non-electronic circuits. For a zero line current I.sub.L, the voltage V.sub.L supplied to the line is: EQU V.sub.L =V.sub.BAT -V.sub.DROP